The present invention pertains to certain transparent, high strength azo pigments and to a method for making these pigments. The pigments of the present invention are characterized by their small particle size (about 10 nanometers to about 40 nanometers mean ultimate particle diameter). It is the heretofore unachieved small mean particle diameter which produces the advantageous high strength and high transparency which characterize the pigment particles of the present invention. Whereas the product of the present invention is a pigment having high strength and high transparency, the process of the present invention concerns a method for making such pigments. This method involves the reaction of a di azo compound with a coupler in the presence of one or more of a group of specific surface active agents.
Prior art azo pigments lack the small particle size and degree of transparency of the pigment of the present invention. Furthermore, prior art methods for making azo pigments do not teach the method of the present invention in that the method of the present invention contemplates the presence of a relatively large amount of one or more of a specific group of surface active agents during the coupling reaction, so that the pigment particles produced during the reaction are exceptionally small in mean ultimate diameter.
With respect to the present invention, the following listing constitutes the closest art known to the inventor on his filing date:
U.S. Pat. No. 3,120,508; Baumgart, Dieter, et al, Improved Pigments for Modern Printing Inks, American Ink Maker, Volume 99, pages 26-42, January, 1989; U.S. Pat. Nos. 4,461,647; 3,775,148; 4,057,463.
Below is a brief discussion of the closest art known to the inventor of the present invention.
U.S. Pat. Nos. 3,120,508 ('508) describes a process for the manufacture of water-insoluble azo-dyestuffs. The '508 patent teaches the use of a cationic surface-active compound during the coupling reaction, the surface-active compound being present in the range of about 1 to about 10 percent on weight of dyestuff. The '508 patent refers to a broad list of surface-active amines and quaternary ammonium compounds, and the '508 patent states that the mean particle size of the resulting pigment particles is 0.147.mu.. Furthermore, the '508 patent states that high tintorial strength and increased temperature stability characterizes the resulting pigments.
In contrast, the product of the present invention is limited to pigment particles having a significantly smaller mean particle diameter than the particles described in the '508 patent. Furthermore, not only does the process of the present invention pertain only to the production of the significantly smaller pigment particles, the process of the present invention requires the use of one or more of a specific and narrower group of surfactants than those listed in the '508 patent. Accordingly, there are major differences between the '508 patent and the subject matter of the present invention.
The Baumgart, et al article (hereinafter Baumgart) describes improved pigments having high color strength, extremely high transparency, low viscosity, outstanding rheological characteristics, full dispersion, high gloss, and relatively small particle size. At least one of the specific pigments mentioned in the Baumgart article is Pigment Yellow 13, a disazo pigment. Although Baumgart describes the characteristics of the pigment product itself, Baumgart in no way describes how this pigment product is produced. Baumgart does state that the pigments are "specially treated" and that the improved pigments contain no resin, but other than this Baumgart nowhere mentions any details of how the pigment product is produced. Note FIG. 5 of Baumgart, in which it is shown that the pigment particles have a mean particle diameter between 0.048 and 0.055.mu..
The product and process of the present invention differ from Baumgart in several ways. First, the mean particle size of the pigment of the present invention (as well as the process for making the pigment of the present invention) is significantly lower than the mean particle size of the pigments referred to in Baumgart's FIG. 5. Secondly, in contrast to Baumgart, Applicant has disclosed a detailed, specific process for making the very fine pigment particles of the present invention, this process comprising the use of one or more, specific group of surface-active agents, and furthermore requiring that the use of these surface-active agents be to at least a minimum (relatively large) amount. In contrast, Baumgart nowhere describes any method for making the pigment particles he discloses.
In conjunction with the Baumgart article is another article (Ott et al) published in American Ink Marker in June of 1989. The Ott et al article, like Baumgart, was written by employees of Hoechst. However, even though the Ott et al article refers to pigmentary particle size which exhibits a high degree of transparency and color strength (see page 42 column 1), the minimum average particle size mentioned in Ott et al is 82 nanometers or 0.082.mu..
U.S. Pat. Nos. 3,775,148 ('148) describes diarylide pigment compositions having high strength and high transparency properties in comparison with conventional diaryl pigments. The '148 patent teaches the incorporation of a small percentage of acid-substituted diarylide dyestuffs into the diarylide pigments, the '148 patent further refers to the addition of amines to the prepared pigment product (i.e. a post-treatment with amine). The '148 patent lists a variety of amines to be used in the invention, including primary, secondary, and tertiary amines, and more specifically to the following amines: Armeen T, Rosin Amine, Duomeen C, Armeen L-11 and Duomeen L-15, Ethomeen, Ethoduomeen, and others. Furthermore, the '148 patent states that amount of amine to be added in the post-treatment will generally lie between 0.1 and 35 percent calculated on the weight of pigment and dyestuff.
The present invention differs from the '148 patent in that the '148 patent nowhere refers to the size of the pigment particles produced, in contrast to the product and process of the present invention, wherein the size of the pigment particles is between 0.010 and 0.040.mu.. Another major difference between the process of the present invention and the '148 patent is that the process of the present invention is concerned with coupling in the presence of a surfactant whereas the '148 patent describes a post-treatment with a surfactant. Another major difference between the process of the present invention and the '148 patent is that the '148 patent refers to a broad group of amines as being useful in the process, while the process of the present invention pertains to the use of a significantly smaller group of surfactants. In fact, many of the dyestuffs mentioned in the '148 patent produce precipitation of the surfactants used in the process of the present invention.
U.S. Pat. Nos. 4,057,463 and 4,461,647 (the '463 and '467 patents) both described processes in which a colored compound is post-treated with a surfactant in order to improve the dispersion of the colored species. Both of these patents pertain to dyestuffs, not pigments. Furthermore, the group of surfactants mentioned in each of these patents is much broader than the group of surfactants utilized in the process of the present invention. Thus, there are major differences between the '647 and '436 patents and the present invention.